a very big night tonight, including the manhunt for the killer of a top law officer gunned down in colorado. the chief of that state's prisons. and was killed on his home on the doorstep of his home. the question is, who did it? we'll bring you the latest. >>> also tonight, children and autism. a new study puts the number at a truly eye-popping 1 in 50. previously a study said 1 in 88. has there been a dramatic increase? dr. sanjay gupta joins us. also tonight, a major break in the biggest art heist ever. precious works valued at half a billion dollars, stolen. we have an exclusive interview with a former guard from the museum where the art was taken. we'll tell you what the fbi now knows and take you inside the daring crime. >>> we begin, though, with that manhunt now a massive search for a brazen killer who walked up to the doorstep of colorado's prison chief, rang the bell and shot tom clemens dead. cas cas casey buy ant joins us. what's the latest, casey? >> reporter: the latest is, anderson, authorities are asking for help from the public. because they don't have a good sense of who th